1. Field
This invention relates generally to testing integrated circuits and more specifically to a circuit and structure at a test point of an integrated circuit for use during testing of the integrated circuit after packaging of the integrated circuit.
2. Related Art
An integrated circuit comprises a die that is typically packaged, or encapsulated, for example, in opaque ceramic, plastic insulation or resin. For a normally encapsulated die, the only coupling of signals between the die and the outside world is with metal pins through the insulation. Therefore, for a normally encapsulated die, a signal that only exists within the packaging and does not appear at one of the pins cannot be readily accessed unless there is an opening in the packaging.
An integrated circuit that is normally packaged using another material may be manufactured using ceramic packaging with an opening in the packaging over the die in order that the die can be accessed so that tests of the integrated circuit can be performed. The functionality of an integrated circuit usually does not change with the type of packaging; therefore, integrated circuits that are normally encapsulated, for example, in plastic can be accurately tested when encapsulated in ceramic. The testing of integrated circuits that are packaged in such a way is called “after package”, or final, testing, as opposed to other testing, such as chip probing that occurs before packaging, i.e., when the integrated circuit is still in a wafer stage of manufacture.
The behavior of some analog circuits in integrated circuits is affected by technology process variations that occur during fabrication. Oscillator and bandgap voltage generator circuits are particularly affected by technology process variations; therefore, internal signals related to these circuits are tested to debug the functionality of these circuits, and some trimming of these circuits is usually necessary.
Most analog circuits within integrated circuits include test structures such as probe pads and metal tweak points for laboratory evaluations. Occasionally, adjustments are necessary on packaged samples of integrated circuits because the packaging procedure itself can alter the results of testing that may have been done at the wafer stage. Some integrated circuits include trimming structures, such as fuses, to allow such adjustments or for laboratory evaluation. An ideal trimming exercise should evaluate each on/off effect of trimming by actually connecting or disconnecting portions of a circuit of the packaged sample. One way to conduct such an exercise is to make cuts with a laser and/or to add metal to implement jumpers, such as by using a focused ion beam (FIB) station that can accurately cut tracks and/or deposit tungsten jumpers and so emulate actual trimming action. However, changes made with a FIB station cannot easily be reversed.
With some known final testing methods, a probe needle must remain in contact with a probe pad on the die of the integrated circuit; therefore, the window of the packaging must disadvantageously remain open during testing. Testing over temperature may be difficult if the packaging is open for any reason, such as the cover to the window of the packaging not being in place because a probe needle must remain in contact with a pad probe on the die of the integrated circuit.